Method for purifying acid chlorides

ABSTRACT

Process for the purification of carbonyl chlorides which have been prepared by reacting carboxylic acids with phosgene or thionyl chloride in the presence of a catalyst adduct, in which the carbonyl chlorides are treated with a hydrohalide of carboxamides of the formula (I)                    
     in which R 1  is hydrogen or C 1 - to C 3 -alkyl; R 2  and R 3  independently of one another are C 1 - to C 4 -alkyl, or R 2  and R 3  together are a C 4 - or C 5 -alkylene chain, and the carbonyl chloride purified in this way is isolated by separation off from the carboxamide hydrohalide phase.

The present invention relates to a process for the purification ofcarbonyl chlorides which originate from the reaction of thecorresponding carboxylic acids with phosgene or thionyl chloride, whichleads to carbonyl chlorides with an improved color number.

Carbonyl chlorides are important intermediates in the synthesis of alarge number of chemical products, in particular pharmaceuticals,cosmetics, surfactants and paper auxiliaries. They can be prepared byreacting carboxylic acids with chlorinating agents, such as PCl₃, POCl₃,SOCl₂, SO₂Cl₂ or COCl₂. Of industrial importance are, in particular, thereactions with thionyl chloride, phosphorus trichloride and phosgene.

As a rule, in the synthesis via phosphorus trichloride, one reactant(carboxylic acid or phosphorus trichloride) is initially introduced, andthe other reactant (phosphorus trichloride or carboxylic acid) is slowlyadded. Where appropriate, the synthesis is carried out in a solutiondiluted with a reaction-inert solvent (e.g. toluene). After removal ofthe phosphorous acid formed, the carbonyl chloride is as a rule purifiedby distillation. The addition of a catalyst is not required.

EP-A-0 296 404 describes the purification of crude carbonyl chlorideswhich originate from the chlorination using phosphorus trichloride, inwhich the reaction products are treated with carboxamide hydrohalides.The crude carbonyl chloride solutions from the phosphorus trichlorideroute differ in composition from those obtainable by the phosgene orthionyl chloride route. For example, the latter have:

(i) a considerably higher content of undesired minor components.

(ii) a varying composition of the minor components, which is influencedby the choice of chlorinating agent.

(iii) supplementary to the varying composition of the minor components,also the presence of degradation and/or secondary products from thecatalyst adducts used.

The use of phosgene or thionyl chloride instead of phosphorustrichloride generally leads to a higher conversion and betterselectivity. Both chlorinating agents additionally have the advantageover phosphorus trichloride that only gaseous byproducts are formed,which either escape in the form of gas during the synthesis or can becompletely expelled by stripping with an inert gas when the reaction iscomplete. Furthermore, phosgene, in particular, is a very good valuechlorinating agent.

Thionyl chloride and, in particular, phosgene are less reactive aschlorinating agents compared with phosphorus trichloride. Thepreparation of carbonyl chlorides by reacting carboxylic acids withthionyl chloride is therefore preferably carried out in the presence ofa catalyst to increase the reaction rate. In the preparation by reactionwith phosgene, a catalyst is always used. Catalyst precursors which aresuitable for both chlorinating agents are N,N-disubstituted formamidesand hydrochlorides thereof, and also pyridine or urea. Overviewsrelating to the chlorination by means of thionyl chloride are given inM. F. Ansell in S. Patai, “The Chemistry of Acyl Halides”, John Wileyand Sons, New York 1972, 35-69 and H. H. Bosshard et al., Helv. Chem.Acta 62 (1959) 1653-1658 and S. S. Pizey, Synthetic Reagents, Vol. 1,John Wiley and Sons, New York 1974, ISBN 853120056, 321-557, inparticular 333-335. Both by the phosgene route and also by the thionylchloride route preference is given to using N,N-disubstitutedformamides. These react with said chlorinating agents to give theVilsmeier salts.

The Vilsmeier salt, the actual reactive chlorinating reagent, reactswith the carboxylic acid or the carboxylic anhydride to give the acidchloride. In the process, formamide-hydrochloride is reformed, which canin turn react with phosgene or thionyl chloride to give the Vilsmeiersalt and passes through further catalyst circuits. The N,N-disubstitutedformamide-hydrochlorides or Vilsmeier salts thereof are not, however,very thermally stable, meaning that it is possible for secondaryreactions to take place above 80 to 90° C.

The preferred use of N,N-disubstituted formamides as catalyst precursorfor the phosgenation of carboxylic acids also emerges from EP-A-0 367050, EP-A-0 452 806, DE-A-4 337 785, EP-A-0 475 137 and EP-A-0 635 473.

As regards the color number, in the chlorination of carboxylic acidsusing phosgene or thionyl chloride, the use of catalysts has an adverseeffect. Although these catalysts are separated off by phase separationfollowing the chlorination, they can, however, remain in the product insmall amounts and lead either themselves or as degradation or secondaryproducts to yellow colorations of the carbonyl chlorides. For thisreason, the carbonyl chlorides prepared via phosgene or thionyl chlorideare generally purified by distillation to give largely colorlessproducts. Such a distillation is not only an energy- and time-consumingoperation, but also harbors a number of further disadvantages. Manylonger-chain carbonyl chlorides cannot be distilled without partialdecomposition. Furthermore, it is known that the distilled products canbecome contaminated as a result of decomposition of the catalyst stillpresent in the distillation bottoms. Relatively large amounts ofaccumulated catalyst residue also represent a safety risk during thedistillation since at elevated temperature there is the risk ofspontaneous decomposition.

A further method of purifying the crude carbonyl chlorides is thetreatment with activated carbons. However, these absorptive purificationsteps are industrially complex and, moreover, are not always successful.In addition, a contaminated solid forms, which has to be subsequentlydisposed of in the correct manner.

It is an object of the invention to develop a process for thepurification of carbonyl chlorides which for the most part originatefrom the reaction of carboxylic acids with phosgene or thionyl chloride,which no longer has the known disadvantages and leads to carbonylchlorides having an improved color number.

Surprisingly, we have found that this object is achieved by thedevelopment of a process for the purification of carbonyl chlorideswhich have been prepared by reacting carboxylic acids with phosgene orthionyl chloride in the presence of a catalyst adduct, which comprisestreating the carbonyl chlorides with a hydrohalide of carboxamides ofthe formula (I)

in which R¹ is hydrogen or a C₁- to C₃-alkyl; R² and R³ independently ofone another are C₁- to C₄-alkyl, or R² and R³ together are a C₄- orC₅-alkylene chain, and isolating the carbonyl chloride purified in thisway by separation from the carboxamide hydrohalide phase.

Contaminated carbonyl chlorides which originate from the reaction ofcarboxylic acids with phosgene or thionyl chloride can be worked up byextraction in high yield and with improved color number by the processaccording to the invention. The term “improved color number” is, in thecase of the first treatment of the crude solutions, a reduction in theAPHA color number to less than 50% of the original value for saturatedcarbonyl chlorides, and a reduction in the iodine color number to lessthan 75% of the original value for unsaturated carbonyl chlorides. Thedeterminations of the APHA color number and of the iodine color numberare described in the standard DIN EN 1557 (March 1997).

The treatment of the crude carbonyl chloride solution can be bothspatially and also temporally separate from the synthesis of the crudesolution. The treatment with a hydrohalide of carboxamides of theformula (I) can thus also be carried out in a different apparatus fromthe synthesis of the carbonyl chloride. Although synthesis and treatmentof the crude carbonyl chloride solution can take place directlyfollowing one another in terms of time, it is also possible that theyare temporally separated by hours, days, months or years, meaning thatinterim storage or transportation of the crude solution is alsoincluded.

For treatment of the crude carbonyl chloride solution by the processaccording to the invention, the solution is admixed, in an apparatus,which can also be identical to the reaction apparatus used above, with ahydrohalide of carboxamides of the formula (I)

in which the substituents have the following meanings:

R¹ is hydrogen or C₁- to C₃-alkyl, specifically methyl, ethyl, propyl or1-methylethyl; particularly preferably hydrogen;

R² and R³ independently of one another are a C₁- to C₄-alkyl,specifically methyl, ethyl, propyl, 1-methylethyl, butyl,1-methylpropyl, 2-methylpropyl or 1,1-dimethylethyl, or together are aC₄- or C₅-alkylene chain, specifically CH₂CH₂CH₂CH₂ or CH₂CH₂CH₂CH₂CH2;particularly preferably methyl.

It is essential that the mutual solubility of the carbonyl chlorides andhydrobalides of the carboxamides (I) is low and that two isolatablephases form.

The amount of hydrohalides of the carboxamides (I) to be added isdependent on various factors, but primarily on the type of carbonylchloride itself and the amount of secondary components present in thecrude carbonyl chloride solution, which in turn are evident from thecoloration. Based on the amount of the carbonyl chloride, from 1 to 80%by weight, preferably from 2 to 60% by weight and particularlypreferably from 5 to 50% by weight of hydrohalide of the carboxamides(I) are in general to be used.

The hydrohalide of the carboxamides (I) used is preferably thehydrochloride, particularly preferably the hydrochloride ofN,N-dimethylformamide. The molar fraction of hydrochloride (as HCl),based on the N,N-dimethylformamide, is in the range between 0.1 and 2.5.Preference is given to using a molar fraction of from 1.0 to 2.0.

The preparation of the carboxamide hydrohalides from carboxamides (I)and hydrohalide can be carried out either before the addition of thecarbonyl chloride or after its addition.

The treatment of the crude carbonyl chloride solution with thehydrohalides of the carboxamides (I) is preferably carried out at atemperature of from −15 to 80° C., preferably −10 to 40° C.,particularly preferably at 0 to 30° C., and a pressure from 0.5 to 5 barabs, preferably 0.8 to 1.2 bar abs, with vigorous mixing. The parametersto be set depend here on the desired residual content of carboxamidehydrohalide in the carbonyl chloride phase and, for each system, is tobe matched to the procedures known to the person skilled in the art. Thetime period depends essentially on the solubility of the undesiredsecondary components in the carboxamide hydrohalide phase and islikewise to be determined for the particular system. Generally, vigorousmixing is carried out for one hour at most.

The treatment can be carried out either batchwise or continuously.

(a) Batchwise Treatment:

In the batchwise treatment, the crude carbonyl chloride solution and thecarboxamide hydrohalide phase are combined in an apparatus, and thesystem is vigorously mixed as described above. Suitable apparatuses are,for example, stirred tank reactors or phase-separating vessels (“mixersettlers”). When mixing is complete, the two phases are separated. Thiscan be carried out in the treatment or mixing apparatus which is alreadybeing used or in a separate apparatus, for example a separating vessel.Generally, the two phases have separated after two hours at most and canthen be isolated.

(b) Continuous Treatment:

In the continuous treatment, the crude carbonyl chloride solution andthe carboxamide hydrohalide phase are fed continuously to a treatment ormixing apparatus. The treatment can be carried out in a known manner instirred tank reactors, batteries of stirred tank reactors, staticmixers, phase-separating vessels (“mixer settlers”) or liquid-liquidextraction columns (see Ullmann's Encyclopedia of Industrial Chemistry,6^(th) edition, 1998, Electronic Release, Liquid-liquid-extraction). Anamount corresponding to the amount of both phases introduced iscontinuously drawn off from the treatment or mixing apparatus. Here, itmust be ensured that the ratio between carbonyl chloride and carboxamidehydrohalide remains virtually constant. The amount removed is fed toanother apparatus, for example a separating vessel for separation of thetwo phases. In this connection, it is also possible for a settling zoneto be inserted between treatment or mixing apparatus and separatingvessel. The two phases can be removed separately from the separatingvessel.

To separate off the carboxamide-hydrohalide-containing phase, it is alsopossible to use suitable filters, such as, for example, coalescencefilters of known design. The separated-offcarboxamide-hydrohalide-containing phase can optionally be reused forthe extraction, the amount of hydrohalide passed over into the carbonylchloride phase advantageously being replaced.

The carbonyl chloride solutions treated in this way have an improvedcolor number compared to untreated solutions and can then either be useddirectly for further synthesis stages or, if required, be subjected tostill further treatment procedures. In this regard, renewed treatmentwith a hydrohalide of carboxamides (I) by the process according to theinvention, distillation or adsorptive purification may be mentionedwithout limitation.

In a further embodiment, the separated-off carboxamide hydrohalide phaseis subsequently used as catalyst precursor for the formation of thecatalyst adduct of phosgene or thionyl chloride and theN,N-disubstituted formamide. For this, the separated-off carboxamidehydrohalide phase is treated with phosgene or thionyl chloride in orderto subsequently use it as a catalyst adduct.

Carbonyl chlorides which can be prepared by the process according to theinvention are, for example, those of the formula (III)

in which R stands for the following radicals:

C₁- to C₃₀-alkyl or their aryl- or cycloalkyl-substituted components:

saturated, straight-chain or branched hydrocarbon radical having from 1to 30 carbon atoms, preferably methyl, ethyl, propyl, 1-methylethyl,butyl, 1-methylpropyl, 2-methylpropyl, 1,1-dimethylethyl, pentyl,1-ethylpropyl, hexyl, heptyl, 1-ethylpentyl, octyl,2,4,4-trimethylpentyl, nonyl, 1,1-dimethylheptyl, decyl, undecyl,dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl,octadecyl, nonadecyl, icosyl, henicosyl, docosyl, tricosyl, tetracosyl,pentacosyl, hexacosyl, heptacosyl, octacosyl, nonacosyl, triacontyl,phenylmethyl, diphenylmethyl, triphenylmethyl, 2-phenylethyl,3-phenylpropyl, cyclopentylmethyl, 2-cyclopentylethyl,3-cyclopentylpropyl, cyclohexylmethyl, 2-cyclohexylethyl,3-cyclohexylpropyl;

C₃- to C₁₂-cycloalkyl or their aryl- or cycloalkyl-substitutedcomponents:

monocyclic, saturated hydrocarbon radical having from 3 to 12 ringcarbon atoms, preferably cyclopentyl, cyclohexyl;

C₂- to C₃₀-alkenyl or their aryl- or cycloalkyl-substituted components:

unsaturated, straight-chain or branched hydrocarbon radical having from1 to 30 carbon atoms and 1 to 5 double bonds at any position, preferably2-propenyl, 3-butenyl, cis-2-butenyl, trans-2-butenyl,cis-8-heptadecenyl, trans-8-heptadecenyl, cis,cis-8,11-heptadecadienyl,cis,cis,cis-8,11,14-heptadecatrienyl;

C₃- to C₁₂-cycloalkenyl or their aryl- or cycloalkyl-substitutedcomponents:

monocyclic, unsaturated hydrocarbon radical having from 3 to 12 ringcarbon atoms and 1 to 3 double bonds at any position, preferably3-cyclopentenyl, 2-cyclohexenyl, 3-cyclohexenyl, 2,5-cyclohexadienyl;

C₂- to C₃₀-alkynyl or their aryl- or cycloalkyl-substituted components:

unsaturated, straight-chain or branched hydrocarbon radical having from1 to 30 carbon atoms and 1 to 3 triple bonds at any position, preferably3-butynyl, 4-pentynyl;

C₄- to C₃₀-alkenynyl or their aryl- or cycloalkyl-substitutedcomponents:

unsaturated, straight-chain or branched hydrocarbon radical having from1 to 30 carbon atoms, 1 to 3 triple bonds and 1 to 3 double bonds at anyposition.

Using the process according to the invention, it is also possible toprepare mixtures of said carbonyl chlorides. Non-limiting examples whichmay be mentioned are mixtures comprising C₈- to C₁₈-carbonyl chlorides,which are traded under the trivial names “carboxylic acid chloride”,“tallow fatty acid chloride”, “coconut fatty acid chloride” and “oleicacid chloride”.

Particular preference is given to preparing carbonyl chlorides of theformula (III) by the process according to the invention in which Rstands for the following radicals:

C₁- to C₃₀-alkyl or their aryl- or cycloalkyl-substituted components:

saturated, straight-chain or branched hydrocarbon radical having from 1to 30 carbon atoms, preferably methyl, ethyl, propyl, 1-methylethyl,butyl, 1-methylpropyl, 2-methylpropyl, 1,1-dimethylethyl, pentyl,1-ethylpropyl, hexyl, heptyl, 1-ethylpentyl, octyl,2,4,4-trimethylpentyl, nonyl, 1,1-dimethylheptyl, decyl, undecyl,dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl,octadecyl, nonadecyl, icosyl, henicosyl, docosyl, tricosyl, tetracosyl,pentacosyl, hexacosyl, heptacosyl, octacosyl, nonacosyl, triacontyl,phenylmethyl, diphenylmethyl, triphenylmethyl, 2-phenylethyl,3-phenylpropyl, cyclopentylmethyl, 2-cyclopentylethyl,3-cyclopentylpropyl, cyclohexylmethyl, 2-cyclohexylethyl,3-cyclohexylpropyl;

C₂- to C₃₀-alkenyl or their aryl- or cycloalkyl-substituted components:

unsaturated, straight-chain or branched hydrocarbon radical having from1 to 30 carbon atoms and 1 to 5 double bonds at any position, preferably2-propenyl, 3-butenyl, cis-2-butenyl, trans-2-butenyl,cis-8-heptadecenyl, trans-8-heptadecenyl, cis,cis-8,11-heptadecadienyl,cis,cis,cis-8,11,14-heptadecatrienyl; and mixtures thereof.

Very particularly preferably, the process according to the invention isused to prepare acetyl chloride (R=methyl), propionyl chloride(R=ethyl), butyryl chloride (R=propyl), valeryl chloride (R=butyl),isovaleryl chloride (R=2-methylpropyl), pivaloyl chloride(R=1,1-dimethylethyl), caproyl chloride (R=pentyl), 2-ethylbutyrylchloride (R=1-ethylpropyl), enanthyl chloride (R=hexyl), capryloylchloride (R=heptyl), 2-ethylhexanoyl chloride (R=1-ethylpentyl),pelargonoyl chloride (R=octyl), isononanoyl chloride(R=2,4,4-trimethylpentyl), capryl chloride (R=nonyl), neodecanoylchloride (R=1,1-dimethylheptyl), lauroyl chloride (R=undecyl), myristoylchloride (R=tridecyl), palmitoyl chloride (R=pentadecyl), stearoylchloride (R=heptadecyl), oleoyl chloride (R=cis-8-heptadecenyl),linoleoyl chloride (R=cis,cis-8,11-heptadecadienyl), linolenoyl chloride(R=cis,cis,cis-8,11,14-heptadecatrienyl), arachidoyl chloride(R=nonadecyl) and behenoyl chloride (R=henicosyl) and mixtures thereof.

The carboxylic acids according to formula (III) to be usedadvantageously for the process according to the invention arise from theabove-described definitions for R.

In the preparation of the crude carbonyl chloride solution, the catalystused is a catalyst adduct which originates from the reaction of phosgeneor thionyl chloride with an N,N-disubstituted formamide. The latter,which is also referred to as a catalyst precursor, is defined by theformula (II)

in which R⁴ and R⁵ independently of one another are a C₁- to C₄-alkyl,specifically methyl, ethyl, propyl, 1-methylethyl, butyl,1-methylpropyl, 2-methylpropyl and 1,1-dimethylethyl, or together are aC₄- or C₅-alkylene chain, specifically CH₂CH₂CH₂CH₂ or CH₂CH₂CH₂CH₂CH₂.Preference is given to using N,N-dimethylformamide.

The preparation of the carbonyl chlorides to be used in the processaccording to the invention from the reaction of carboxylic acids withphosgene or thionyl chloride is carried out by prior art processes whichare generally known.

The formation of the catalyst adduct can either be carried out in theapparatus in which the chlorination is carried out, or else upstream inanother apparatus. In the last-mentioned case, a certain amount of theN,N-disubstituted formamide is introduced into a separate apparatus,mixed with the desired amount of phosgene or thionyl chloride and thenpassed to the actual reaction apparatus. In the first-mentioned case,the procedure described is carried out directly in the reactionapparatus.

The reaction of the carboxylic acids with phosgene or thionyl chloridein the presence of the catalyst adduct described can be carried outeither batchwise or continuously. In the phosgene variant, a molar ratiobetween the catalyst adduct and the carboxylic acid of from 0.05 to 2.0,preferably from 0.1 to 1.0, particularly preferably from 0.1 to 0.3, isto be set, and in the thionyl chloride variant, a molar ratio of from0.001 to 0.05, preferably from 0.001 to 0.01, is to be set. In bothvariants, the reaction is carried out at temperatures between 20 and100° C., preferably between 30 and 80° C., particularly preferablybetween 30 and 70° C., and a pressure between 0.5 and 2.0 bar abs,preferably from 0.8 to 1.2 bar abs, particularly preferably atatmospheric pressure. The total amount of phosgene or thionyl chlorideadded is from 1.0 to 2.0 of the molar amount of the carboxylic acidused, preferably from 1.0 to 1.3 of the molar amount of the carboxylicacid used.

(a) Batchwise Preparation:

In the batchwise preparation, the reaction mixture, consisting of acarboxylic acid and the catalyst adduct, prepared from phosgene orthionyl chloride and the N,N-disubstituted formamide of the formula(II), is introduced into a reaction apparatus, for example a stirredtank reactor, and brought to the reaction temperature and, wherenecessary, to the reaction pressure. Then, the desired amount of liquidor gaseous phosgene or thionyl chloride is added over a certain periodof time. The time requirement for the addition of the chlorinating agentdepends on the rate of the reaction and can generally be limited to afew hours. The reaction solution is then generally left to stand for 1to 2 hours, and the two phases are separated from one another. As arule, the carbonyl-chloride-containing phase is the upper phase, and thecatalyst-adduct-containing phase is the lower phase.

(b) Continuous Preparation:

Reaction apparatuses suitable for the continuous procedure are, forexample, stirred tank reactors, batteries of stirred tank reactors orreaction columns operated countercurrently. Using a stirred tankreactor, the carboxylic acid and the catalyst adduct, prepared fromphosgene or thionyl chloride and the N,N-disubstituted formamide of theformula (II), are initially introduced, the desired reaction temperatureand optionally the desired reaction pressure are set, and liquid orgaseous phosgene or thionyl chloride is added. After an amount ofchlorinating agent approximately equivalent to the carboxylic acid hasbeen introduced, the simultaneous introduction of carboxylic acid andcatalyst adduct, and also an amount of phosgene or thionyl chloridewhich is essentially equimolar to the introduced carboxylic acid, isstarted. An amount of the reaction volume corresponding to theintroduced reactants is drawn off from the reaction apparatus, forexample via a level control, and passed to a separating vessel. In theseparating vessel, the carbonyl chloride of the formula (III), as upperphase, can be continuously drawn off, and the catalyst adduct, as thelower phase, can be continuously returned to the reactor. In carryingout the reaction, it must be ensured that the chlorinating agententrained by the reaction exit gases is replaced by introducingadditional chlorinating agent.

The catalyst phase can be separated off at temperatures of from −15° C.to 40° C., preferably −10 to 30° C., particularly preferably −5 to 20°C. The upper, carbonyl-chloride-containing phase is referred to below asthe crude carbonyl chloride solution. To separate off the catalyst phaseit is also possible to use suitable filters, such as, for example,coalescence filters of known design.

The process according to the invention does not rule out the possibilityof also adding carboxylic acids of another origin to a low extent.

Preferably, the carbonyl chlorides according to Formula (III) used forthe purification are for the most part obtained from the reaction of thecorresponding carboxylic acids with phosgene in the presence of thecatalyst adduct described.

In a general variant for the batchwise preparation of the crude carbonylchloride solution by reacting the carboxylic acid with phosgene, thecatalyst adduct, which can be obtained by introducing phosgene intoN,N-disubstituted formamide, is initially introduced into a stirred tankreactor, and carboxylic acid is added thereto. After the desiredreaction conditions temperature and optionally pressure have been set,the amount of gaseous or liquid phosgene required for the reaction iscontinuously introduced with stirring over the course of the desiredperiod. When the reaction is complete, the contents of the stirred tankreactor are transferred to a separating vessel for phase separation. Thestirred tank reactor is then available for a further reaction batch.After about 1 to 2 hours the two phases have clearly separated from oneanother. The lower phase, which is generally the catalyst-containingphase, is separated off, and the carbonyl chloride phase, which isreferred to as the crude carbonyl chloride solution, is isolated.

In a general variant for the batchwise purification, the crude carbonylchloride solution is transferred to a stirred tank reactor, carboxamideis added, and the desired amount of hydrohalide is introduced withstirring. In the process, a second phase forms, which consistspredominantly of the carboxamide hydrohalide. Alternatively, it is alsopossible to add carboxamide hydrohalide which has been preparedseparately. In this second, carboxamide-hydrohalide-containing phase,the undesired, color-imparting secondary components of the carbonylchloride are dissolved with stirring. By switching off the stirrer ortransferring the mixture to a further separating vessel the two phasesare separated. The carbonyl-chloride-containing phase can then, ifnecessary, be subjected to further purification, for example by renewedextraction with carboxamide hydrohalide, or to removal of dissolvedhydrohalide, for example by stripping with inert gas, such as, forexample, nitrogen or argon, or by evacuation. Thecarboxamide-hydrohalide-containing phase can optionally be reused forthe extraction. If an N,N-disubstituted formamide is used which isidentical to the catalyst precursor, then charging with phosgene andsubsequent use as catalyst adduct in the carbonyl chloride synthesis canalso take place. At the same time, partial discharge is also possible toremove the accumulated, color-imparting impurities. The dischargedcarboxamide-hydrohalide-containing phase can, furthermore, also bedisposed of or purified by distillation to remove the impurities.

In a further general variant for the continuous preparation of the crudecarbonyl chloride solution by reacting the carboxylic acid withphosgene, the carboxylic acid, recycled catalyst adduct and gaseous orliquid phosgene are continuously fed to a stirred tank reactor under thedesired reaction conditions with stirring. An amount corresponding tothe introduced amount is continuously drawn off from the stirred tankreactor and passed to a separating vessel. From this, thecatalyst-containing phase, which is usually at the bottom, iscontinuously separated off and returned to the stirred tank reactor. Forremoval of impurities, it is advantageous to bleed out a small fractionbetween 1 and 10% by weight and to replace it by fresh catalystprecursor. The crude carbonyl chloride solution is likewise continuouslydrawn off from the separating vessel.

In a further general variant for the continuous purification, the crudecarbonyl chloride solution is passed for extraction to a battery ofstirred tank reactors, with, for example, two stirred tank reactors. Inparallel to the introduction of the crude carbonyl chloride solution,recycled carboxamide hydrohalide is added to the first stirred tankreactor. To replace discharged hydrohalide, gaseous hydrohalide isintroduced into the first stirred tank reactor. After passing throughthe individual battery stages, the runoff from the last stirred tankreactor passes to a separating vessel, where the two phases separatefrom one another. The carbonyl-chloride-containing phase is continuouslydrawn off and further processed as described under the batchwisevariant. The carboxamide-hydrohalide-containing phase is likewisecontinuously drawn off and returned to the first stirred tank reactor.To remove the extracted impurities, it is advantageous to bleed out afraction between 1 and 20% by weight and replace it with freshcarboxamide or its hydrohalide.

An essential feature in the treatment of the crude carbonyl chloridesolution according to the invention and described above is thesurprising effect that precisely the color-imparting components areconsiderably more soluble in the carboxamide-hydrohalide-containingphase than in the carbonyl-chloride-containing phase.

The process according to the invention leads, mainly as a result of asingle extraction, to a significant reduction in color number, meaningthat the carbonyl chlorides purified in this manner can generally beused for subsequent reactions without distillation or adsorptivetreatment. The process according to the invention can be carried outvery effectively and economically. By avoiding the distillation which iscustomary according to the prior art, both investment and energy costsare saved, and also as a rule a higher yield of purified carbonylchloride is achieved. For distillation-sensitive carbonyl chlorides, theprocess according to the invention opens up the possibility of aneconomical synthesis on an industrial scale.

EXAMPLES Synthesis 1: Preparation of N,N-Dimethylformamide Hydrochloride

The synthesis of N,N-dimethylformamide hydrochloride is described in I.S. Kislina et al., Russ. Chem. Bl., EN, 43(9), 1994, 1505-1507. 365.5 g(5.0 mol) of N,N-dimethylformamide (DMF) were introduced into a stirredapparatus and heated to 45° C. Then, with stirring, gaseous HCl isintroduced until the onset of an amount of exit gas. This gave a clear,colorless liquid which corresponded, according to the elementalanalysis, to a composition of DMF*2HCl.

Process 1: Batchwise Preparation of the Crude Carbonyl Chloride Solutionvia Phosgene

For the preparation of the crude carbonyl chloride solutions by abatchwise process, in each case 2 to 5 mol of the correspondingcarboxylic acids were introduced into a stirred apparatus, and 10 to 50mol %, based on the carboxylic acid used, of N,N-dimethylformamide wereadded thereto. The reaction solution was brought, with stirring, to atemperature of from 25 to 45° C., and gaseous phosgene was introducedunder atmospheric pressure. After the stoichiometric amount (based onthe amount of carboxylic acid introduced) of phosgene, including thedesired excess, had been introduced, the further introduction wasstopped, and the system was left to stand for 2 hours with the stirrerswitched off. Following separation of the two phases, the crude carbonylchloride solution, as the upper phase, was isolated and the APHA colornumber was determined.

Process 2: Continuous Preparation of the Crude Carbonyl ChlorideSolution via Phosgene

For the preparation of the crude carbonyl chloride solutions by acontinuous process, in each case 0.75 mol/h of the correspondingcarboxylic acids, 30 g/h of recycled catalyst adduct, and 0.75 to 0.80mol/h of gaseous phosgene were introduced into a stirred apparatus at atemperature of 45° C. and a pressure of 1 bar abs. By means of the levelcontrol which was present, the corresponding amount of reaction mixturewas drawn off and passed to a separating vessel. The lower,catalyst-containing phase was recycled. The upper,carbonyl-chloride-containing phase was isolated as crude carbonylchloride solution and the APHA color number was determined.

Example 1

Purification of Lauroyl Chloride

Lauric acid and phosgene were used to prepare, by the batchwise process1, a crude lauroyl chloride solution having a color number of 268 APHA.200 g of this product were stirred vigorously with 50 g of DMFhydrochloride from synthesis 1 in a stirred apparatus, and then thephases were separated. The lauroyl chloride phase was stripped untilHCl-free using nitrogen. The color number was then only 48 APHA.

Example 2

Purification of Coconut Fatty Acid Chloride

Coconut fatty acid (trade name HK 8-18, Henkel), which consistsessentially of lauric acid and myristic acid, and phosgene were used toprepare, by the batchwise process 1, a crude coconut fatty acid chloridesolution having a color number of 399 APHA. 200 g of this product werestirred vigorously with 50 g of DMF hydrochloride from synthesis 1 in astirred apparatus and then the phases were separated. The coconut fattyacid chloride phase was stripped until HCl-free using nitrogen. Thecolor number was then only 64 APHA.

Example 3

Purification of Pelargonoyl Chloride (Nonanoyl Chloride)

Pelargonic acid (nonanoic acid) and phosgene were used to prepare, bythe continuous process 2, a crude pelargonoyl chloride solution having acolor number of 301 APHA. 90 g of this product were stirred vigorouslywith 10 g of DMF hydrochloride from synthesis 1 in a stirred apparatus,and then the phases were separated. The pelargonoyl chloride phase wasstripped until HCl-free using nitrogen. The color number was then only55 APHA. Repetition of the extraction using a further 10 g of DMFhydrochloride from synthesis 1 led to a colour number of 36 APHA. Aftera third extraction which was carried out analogously, the color numberwas then only 30 APHA.

Example 4

Purification of Pelargonoyl Chloride (Nonanoyl Chloride)

Pelargonic acid (nonanoic acid) and phosgene were used to prepare, bybatchwise process 1, a crude pelargonoyl chloride solution having acolor number of 118 APHA. 90 g of this product were stirred vigorouslywith 10 g of DMF hydrochloride from synthesis 1 in a stirred apparatus,and then the phases were separated. The pelargonoyl chloride phase wasstripped until HCl-free using nitrogen. The color number was then only50 APHA. Repetition of the extraction with a further 10 g of DMFhydrochloride from synthesis 1 led to a color number of 48 APHA. After athird extraction, carried out in an analogous manner, the color numberwas 45 APHA.

Example 5

Purification of Pivaloyl Chloride

Pivalic acid and phosgene were used to prepare, by the continuousprocess 2, a crude pivaloyl chloride solution having a color number of361 APHA. 200 g of this product were vigorously stirred with 50 g of DMFhydrochloride from synthesis 1 in a stirred apparatus, and then thephases were separated. The pivaloyl chloride phase was stripped untilHCl-free using nitrogen. The color number was then only 35 APHA.

Example 6

Purification of Pivaloyl Chloride (Repetition)

Pivalic acid and phosgene were used to prepare, by the continuousprocess 2, a crude pivaloyl chloride solution having a color number of409 APHA. 200 g of this product were vigorously stirred with 50 g of DMFhydrochloride from synthesis 1 in a stirred apparatus, and then thephases were separated. The pivaloyl chloride phase was stripped untilHCl-free using nitrogen. The color number was then only 83 APHA.

Since, compared to Example 5, in the present example the starting crudesolution had a higher color number, a purified solution with a highercolor number was also obtained. The depletion of the color componentsis, however, comparable in the two examples.

Example 7

Purification of Oleoyl Chloride

Oleic acid and phosgene were used to prepare, by continuous process 2, acrude oleoyl chloride solution having an iodine color number of 38. 200g of this product were vigorously stirred with 50 g of DMF hydrochloridefrom synthesis 1 in a stirred apparatus, and then the phases wereseparated. The oleoyl chloride phase was stripped until HCl-free usingnitrogen. The iodine color number was then only 26.

Example 8

Purification of Palmitoyl Chloride

Palmitic acid and phosgene were used to prepare, by batchwise process 1,a crude palmitoyl chloride solution having a color number of 202 APHA.20 ml of this product were vigorously stirred with 5 ml of DMFhydrochloride from synthesis 1 in a stirred apparatus, and then thephases were separated. The palmitoyl chloride phase was stripped untilHCl-free using nitrogen. The color number was then only 82 APHA.

Example 9

Purification of Pelargonoyl Chloride (Nonanoyl Chloride)

0.4 g (0.005 mol) of N,N-dimethylformamide were added to 158 g (1.0 mol)of pelargonic acid, and the mixture was heated to 50° C. At 50° C., atotal of 125 g (1.05 mol) of thionyl chloride were added dropwise overthe course of 45 minutes. After a post reaction time of 30 minutes at50° C., nitrogen was passed through the mixture at 50° C. for 1 hour,and sulfur dioxide, hydrogen chloride gas and unreacted thionyl chloridewere stripped out. The pale yellow product had a color number of 113APHA and, according to GC analysis, comprised 99.5 area % of pelargonoylchloride.

20 ml of the product were vigorously stirred with 5 ml of DMFhydrochloride from synthesis 1 in a stirred apparatus, and then thephases were separated. The pelargonoyl chloride phase was stripped untilHCl-free using nitrogen. The color number was then only 37 APHA.

The examples show for carbonyl chlorides from both synthesis routes, viaphosgene and via thionyl chloride, that irrespective of the type ofcarboxylic acid, i.e. irrespective of whether the carboxylic acid issaturated or unsaturated, or straight-chain or branched, the colornumber can be significantly reduced as a result of the treatment(extraction) according to the invention. Repeated extraction leads to afurther reduction in the color number. The carbonyl chlorides obtainedin the examples can be used in subsequent syntheses without furtherpurification steps.

We claim:
 1. A process for the purification of carbonyl chlorides which have been prepared by reacting carboxylic acids with phosgene or thionyl chloride in the presence of a catalyst adduct, which comprises treating the carbonyl chlorides with a hydrohalide of carboxamides of the formula (I)

in which R¹ is hydrogen or a C₁- to C₃-alkyl; R² and R³ independently of one another are C₁- to C₄-alkyl, or R² and R³ together are a C₄- or C₅-alkylene chain, the mutal solubility of the carbonyl chlorides and the hydrohalides of the carboxamides (I) being low, and isolating the carbonyl chloride purified in this way by separation from the carboxamide hydrohalide phase.
 2. A process as claimed in claim 1, wherein, for the treatment of the carbonyl chlorides, an amount of carboxamide hydrohalide of from 1 to 80% by weight, based on the amount of carbonyl chloride employed, is used.
 3. A process as claimed in claim 1, wherein the carboxamide hydrohalide used is N,N-dimethylformamide hydrochloride.
 4. A process as claimed in claim 1, wherein the treatment with the carboxamide hydrohalide is carried out at a temperature of from −15 to 80° C. and a pressure of from 0.5 to 5.0 bar abs.
 5. A process as claimed in claim 1, wherein, as catalyst precursor for the catalyst adduct to be formed, an N,N-disubstituted formamide of the formula (II) is used

in which R⁴ and R⁵ independently of one another are C₁- to C₄-alkyl, or R⁴ and R⁵ together are a C₄- or C₅-alkylene chain.
 6. A process as claimed in claim 1, wherein the catalyst precursor according to the formula (II) used is N,N-dimethylformamide.
 7. A process as claimed in claim 3, wherein the N,N-dimethylformamide hydrochloride, after it has been used as treatment agent, is used as catalyst precursor in the carbonyl chloride synthesis.
 8. A process as claimed in claim 1, wherein most of the carbonyl chlorides used originate from the reaction of carboxylic acids with phosgene in the presence of a catalyst adduct.
 9. A process as claimed in claim 1, wherein the carbonyl chlorides to be purified are acetyl chloride, propionyl chloride, butyryl chloride, valeryl chloride, isovaleryl chloride, pivaloyl chloride, caproyl chloride, 2-ethylbutyryl chloride, enanthyl chloride, capryloyl chloride, 2-ethylhexanoyl chloride, pelargonoyl chloride, isononanoyl chloride, capryl chloride, neodecanoyl chloride, lauroyl chloride, myristoyl chloride, palmitoyl chloride, stearoyl chloride, oleoyl chloride, linoleoyl chloride, linolenoyl chloride, arachidoyl chloride and behenoyl chloride, and mixtures thereof. 